FIG. 1 is a perspective view illustrating a conventional speaker system, and FIG. 2 is a diagram showing frequency curves of sound pressure level in the conventional speaker system of FIG. 1. In FIG. 2, the vertical axis shows sound pressure level, and the horizontal axis shows frequency.
In FIG. 1, speaker 1 is secured in box 3, and sound waves are radiated from diaphragm 5 of speaker 1. Suppose that the solid line (at 0 degrees) indicates the central axis of speaker 1, and the broken line and the single-dot broken line indicate the directions at angles of 30 and 60 degrees from the central axis, respectively. These three types of lines in FIG. 2 respectively indicate the sound pressure levels at the corresponding angles. As is apparent from FIG. 2, the larger the angle from the central axis, at 30 and 60 degrees, and the higher the frequency component, the less the sound pressure level. Thus, since the sound pressure level is what makes a particular frequency audible, FIG. 2 shows that the higher frequencies become inaudible at greater angles.
FIG. 3 is a cross-sectional view showing the conventional speaker system of FIG. 1 and will now be used for explaining the reason of the above-mentioned phenomenon. The sound waves radiated from each concentric part of diaphragm 5 (for example, points P1 and P2) are at the same distance from diaphragm 5 on the central axis. However, in the diagonal direction of 30 and 60 degrees, the distances from each concentric part are different. Therefore, in the high frequency band of sound waves which has a short wavelength, since half of the wavelength becomes equal to the difference D of these distances, the sound waves become weaker due to mutual cancelling out.
For this reason, in the known speaker system, the sound pressure level is reduced in the direction having an angle from the central axis of the speaker. Moreover, the greater such angle and the higher the frequency band of sound waves, the more noticeably the sound pressure level is reduced.